1. Field of the Invention
The present invention relates to an intake air amount detecting device for an internal combustion engine which is equipped with an electronically controlled fuel injection system.
2. Description of the Prior Art
To supply an internal combustion engine with an air-fuel mixture at a controlled, predetermined fixed air-fuel ratio is very effective to purify the engine exhaust gases. It is therefore necessary to accurately detect the amount of intake air into an internal combustion engine to determine a corresponding supply of a fuel to the engine.
The prior art intake air amount detecting system was so arranged as to indirectly detect a volumetric amount of intake air either on the basis of the engine speed and the intake vacuum or on the basis of the engine speed and the degree of the throttle valve opening. Thus, manufacturing tolerance of engine, deterioration of engine, improper adjustment of intake and exhaust valves and variation in the operation of air cleaner with the lapse of time greatly adversely affect the measurement of intake air amount with resultant large errors in the measured amount of intake air. In addition, the measurement of volumetric amount of intake air needs absolute pressure-based compensating calculation which makes the intake air detecting system complicated and expensive.
U.S. Pat. No. 3,975,951 issued Aug. 24, 1976 to Tokio Kahama et al discloses an improved intake air amount detecting system for an internal combustion engine which comprises a temperature detector housing disposed in an intake pipe of the internal combustion engine. The detector housing defines therein first and second air passages extending axially of the intake pipe. First and second temperature detecting elements are disposed in the air passages, respectively. An electric heater is disposed in the first air passage to heat the air flowing therethrough so that a temperature difference is produced between the flows of air through the first and second air passages. The temperature difference is detected by the first and second temperature detecting elements, while the voltage applied to the electric heater is controlled to adjust the heat generation of the heater so that the temperature difference is substantially constant. The controlling voltage is related to the amount by weight of intake air and thus is utilized as an electrical signal representative of intake air amount. The disclosure in the U.S. patent referred to is incorporated herein by reference.
The system disclosed in the U.S. patent is operable to accurately and directly measure the amount by weight of intake air flowing through an intake pipe of an associated internal combustion engine. From a practical point of view, however, the intake air detecting system is required to have a sufficient endurance to withstand not only the vibration and oscillation caused by the engine but also the vibration and oscillation produced during the operation of a vehicle on which the engine is mounted.